White backdrop, models front is metered at f8 the background is metered at f11 so what should the hairlight be metered at ? And should the hair light be right over the models head ? This is for a catalog shoot.

The only thing you need to consider when lighting for hair is the angle of refraction. There is argument on blonde vs brunette but f-8 is f-8. I always meter hair light the same as main. However I might angle the hair light more on brunettes vs blondes. So a blonde will have it more above and brunette might have above on a boom and back at a 45 degree angle to the axis of lens. Lens with a hood. This is where room in a studio helps.

If you are shooting high key, don't blow the hair out with some techniques saying to overexpose the hair when the hair is dark. I know some will argue light absorption with dark hair but angle of refraction always takes care of this and always produces nice results and you don't run the risk of blowing the refraction highlights out.

BTW. I am just not sure you need a hair light with high key background. You also have to consider a blonde at angle of refraction might bleed the highlights and deform the hair. You really need to chimp or polaroid it to find out exactly your desired results.

[quote=
BTW. I am just not sure you need a hair light with high key background. You also have to consider a blonde at angle of refraction might bleed the highlights and deform the hair. You really need to chimp or polaroid it to find out exactly your desired results.

jt[/QUOTE]

I want to make the images not look so flat. But I guess with a white background you don't have a choice.